NEW YORK (AP) ¿ Shares of ZymoGenetics Inc. dipped Tuesday after the company said additional studies are likely needed to gain European regulatory approval of its only marketed product, Recothrom. Sales of the drug, which is used to control bleeding during surgical procedures, climbed to $8.5 million from $1.8 million during the third quarter. While approved in the U.S., it is not yet approved in Europe. Seattle-based ZymoGenetics licensed all rights outside of the U.S. to Germany-based Bayer in 2007. In a Securities and Exchange Commission filing Monday, ZymoGenetics said Bayer pulled its application with European regulators as a response to indications that the drug candidate would take an additional study to gain approval. The Committee for Medicinal Products for Human Use said Bayer's submission did not meet the necessary standards. The stock fell 13 cents, or just under 2 percent, to $6.61 in afternoon trading. ZymoGenetics shares have traded between $2.55 and $7.31 over the last 52 weeks.